The present invention is broadly concerned with reference electrodes, and the reference electrode portion of combination electrodes, which are employed to provide the stable reference potentials required by a variety of electroanalytical techniques, such as ion-selective electrode measurements, controlled potential coulometry, polarography, and the like. More particularly, the present invention is concerned with a reference electrode and the reference electrode portion of a combination electrode, each having an improved removable and replaceable junction.
A reference electrode most frequently is used in conjunction with an ion-selective electrode, either separately or in combination, to measure the activity (which is a function of concentration) of a given ion in a sample solution. Consequently, the discussion which follows primarily relates to such use. It is to be understood, however, that such discussion is not intended to in any way limit the spirit or scope of the present invention.
The two electrodes, i.e., the reference electrode and the ion-selective electrode, both of which are immersed in the sample solution, typically are connected to a means for measuring the potential difference between the two electrodes, e.g., an electrometer. The reference electrode provides a constant electromotive force or potential against which the potential of the ion-selective electrode is compared. The latter potential consists of a constant component from the electrochemical half-cell of the ion-selective electrode and a variable component which is the potential across the sensing membrane and which is dependent upon the activity (concentration) of the ion being measured. The variable component, then, is readily correlated with ion activity (concentration) by known means. To give accurate results, the potential of the reference electrode should not change with the composition of the sample.
The reference electrode is designed to be minimally sensitive to changes in the external, sample ionic environment. It consists of at least three components: (1) a half-cell electrode (typically a silver-silver chloride mixture), (2) a half-cell electrolyte (typically 4M potassium chloride solution saturated with silver ions), and (3) a reference junction. The half-cell electrode and half-cell electrolyte constitute an electrochemical half-cell having a known, stable, constant electrical potential. Direct physical, and therefore electrical, contact between the half-cell electrolyte and the sample solution is established through the reference junction which usually consists of a porous ceramic plug, metal or asbestos fiber bundle, sintered plastic, or like means of achieving a fluid mechanical leak.
As used herein, the term "half-cell electrode" means the solid-phase, electron-conducting contact with the half-cell electrolyte, at which contact the half-cell oxidation-reduction reaction occurs which establishes the stable potential between the half-cell electrolyte and the contact.
Because the junction electrolyte and the measured sample usually differ in ionic strength and transference, a "liquid junction potential" typically develops across the reference junction. Variation in this junction potential from sample to sample is a source of error in electrode measurements, and one goal of reference electrode technology is to make the junction potential as small, stable, and reproducible as possible. But the reference junction, for various reasons usually involving clogging, can become wholly or partly inoperable. Clogging of junction pores by foreign materials disrupts the direct physical contact which is required to establish a stable, reproducible liquid junction potential between the internal and measured solutions. Also, clogging typically introduces fixed ionic charge into the junction, which causes an anomalous increase in junction potential in low-ionic-strength measurements. Also, in many reference electrode designs, the internal filling solution flows out of the reference electrode into the measured solution. I have found that this flow generally results in faster and more accurate response, since the flow serves to flush the previously measured solution more rapidly from the junction and also serves to increase the ionic strength at the junction surface, thereby reducing anomalies due to fixed space-charge in the junction. Clogging blocks this beneficial flow of junction electrolyte, leading to slower, less accurate measurements. Finally, clogging increases the electrical resistance of the junction, which causes a proportionate increase in the electrical noise of the measurement. Thus, typical symptoms of a clogged junction include slow, erratic, noisy, and often erroneous response.
Junction clogging may arise from a variety of sources, both extrinsic and intrinsic. For example, the proteins and lipids present in many measured samples tend, because of electrostatic and hydrophobic forces, to bind to and permeate many junction materials. Also, certain components of the filling solution tend to precipitate upon coming into contact with the measured solution within the junction. For example, AgCl and Ag.sub.2 S tend to precipitate within the junction of Ag/AgCl reference electrodes immersed in dilute chloride- and sulfide-containing samples, respectively.
In the prior art, failure of the reference junction has usually meant replacement of the entire reference electrode, an expensive, undesirable solution where the reference junction is often the least expensive component of the reference electrode. Attempts to replace the reference junction by the laboratory practitioner have usually ended in failure, since in most high-quality electrodes, the junction is permanently fused or cemented to the electrode body. Even in electrode designs where the junction is held within an orifice by friction or pressure alone, the junction is typically too fragile to withstand the forces required for removal or insertion. Finally, ven if the junction could be removed by force, some portion of the porous junction material would have to extend beyond the electrode body to allow traction. But, I have found that protrustion of the porous material into the measured solution may contribute somewhat to a low and inaccurate response by introducing an element of spherical rather than planar diffusion. A flat junction surface is preferable, which is incompatible with protrusion of the porous member.
Turning now to the known prior art, a 1979 Graphics Controls catalog shows a commercially available renewable junction electrode. A new junction is created by pulling the threadlike woven fiber junction to expose a fresh increment. When the built-in supply of woven fiber junction is exhausted, the entire electrode must be replaced.
U.S. Pat. No. 4,282,081 discloses a double junction reference electrode having a removable sealing plug at the lower end of the lower or external junction electrolyte compartment, which plug contains a conduit extending axially through the plug and providing a flow-restrictive fluid permeable path between the lower compartment and a test system external of the electrode, i.e., an outer liquid junction. The conduit or outer junction preferably is made of porous ceramic. Thus, the outer junction is replaceable only by replacing a portion of the electrode, i.e., the removable sealing plug. According to the patent, the electrode components are separable at threaded junctions to allow easy access to the internal chambers for thorough cleaning and convenient refilling. There is no recognition in the patent regarding outer junction clogging or the problems associated therewith, and no indication that an ability to replace the outer junction is advantageous. Furthermore, such replacement, if actually done, requires replacement of the removable sealing plug which contains the outer junction. Moreover, the need to replace the entire plug prevents the use of such a concept in a combination electrode, an inherent disadvantage. As already noted, however, cleaning and refilling are the only reasons cited for dismantling the electrode.
U.S. Pat. No. 2,058,761 relates to an apparatus for testing acidity. The apparatus includes two electrodes, one of which comprises a glass tube open at both ends but tapered inwardly at the lower end to constitute a seat portion of frustro-conical shape, in which lower end there rests a frustro-conical plug. While the plug prevents practically all leakage of fluid from the tube, there always is present a thin film of solution between the surfaces of the plug and the tube to provide ready conduction of current between the solution to be tested and the electrolyte contained in the tube. The composition of the plug is not specified.
U.S. Pat. No. 3,530,056 discloses a flexible liquid junction which includes a flexible, electrically insulating sheath, within which is disposed at one end an electrically insulating porous plug and a wettable wicking which extends from the plug to the other end of the sheath. The plug can be a porous ceramic. The patent neither teaches nor implies that such flexible liquid junction is removable and replaceable.